Transmission systems that take in DS3 of STS (Synchronous Optical Network Transport Signal) signals in large numbers often employ a 1-for-n or m-for-n protection scheme whereby a number of interface cards (n) are protected by one or m spare interface cards. The incoming signals and outgoing signals can be routed to the spare card in a variety of ways. The most common is to pass the signal first to a switch card, which can route the signals either to the working cards or to the spare cards. One problem with this approach is that if the switch card is removed from the system, the paths to both the working and the spare card are broken and the signals are lost. Some systems employ special connectors which, when the card is removed, connect the signal paths. Unfortunately, these connectors have proven unreliable in service.
Another problem is that the output circuits on the working cards cannot be tested without affecting the output signals. If an incoming signal fails outside the system, the system of course cannot protect against it. However, conventional testing methods do not know whether the failure is external until they switch the signal to the spare card to try to protect it. This switching event may affect other signals. The performance of transmission systems is sometimes gauged by customers by the number of switches that the transmission systems make in changing in and out working interface cards. Conventional testing methods are undesirable because of the unnecessarily large number of switching events.
A need therefore exists for an ability to bridge onto a high-speed telecommunication signal to test it without affecting the signal, and to provide a monitoring and testing method which does not cause countable switching events in the working interface cards.